The administration of testosterone (T) to normal men suppresses spermatogenesis and represents a promising reversible form of contraception. However, current hormonal contraceptive regimens for men, including T treatment, show incomplete efficacy, which limits their potential use. A particular problem is variation in the degree of spermatogenic suppression ; e.g., only 50-70% of Caucasian men develop azoospermia (zero sperm count) on current hormonal regimens. The reason for this variation is obscure. The present study in monkeys is designed to elucidate intrinsic differences especially at the testicular levels between azoospermic and oligospermic (<3 million/mL) responders. Nine normal adult male macaques were implanted subcutaneously with T-filled silastic tubes for 20 wk. Hormonal profiles, semen analyses, body weight and testicular volumes were monitored every 2 wk during pretreatment (4 wk), treatment (20 wk) and posttreatment (16 wk). As observed for men, sper mato genic suppression was not uniform only 4/9 monkeys became azoospermic. Animals that were azoospermic had decreased testicular size, complete spermatogenic arrest, and markedly reduced levels of follicle stimulating hormone (FSH). Their testicular histology showed a tremendous reduction in the germ cell number. On the other hand, oligospermic animals had relatively larger testis size, were less sensitive to FSH suppression, maintained early germ cell population in the testis, and thus showed a persistence of spermatogenesis. The discrepancies between oligospermic and azoospermic responders are now being analyzed. FUNDING NIH grant RR00166 and a grant from the Andrew W. Mellon Foundation.